Converter for changing alternating current into direct current

ABSTRACT

A converter for changing alternating current into direct current by charging a large-capacity capacitor through a full wave rectification circuit which, together with a transformer, is connected to an AC power supply. The output voltage of a voltage divider circuit consisting of the above-mentioned capacitor and a parallel-connected resistor is compared with a predetermined reference voltage by a comparator circuit. Then, if the former is lower than the latter, a gate circuit is triggered to put a selfoscillating circuit into a state where it is ready for oscillation, and the oscillating operation of the above-mentioned self-oscillating circuit is triggered through the gate circuit connected to an intermediate point of a closed loop made up of a rectification circuit for rectifying an AC source voltage and of a part of the aforesaid voltage divider circuit, the output of the self-oscillating circuit providing a trigger signal for a thyristor means connected to the aforementioned transformer.

United States Patent Tatematsu et al. 51 Feb. 29, 1972 [54] CONVERTERFOR CHANGING 3,507,096 4/1970 Hall et al ..321/18 X ALTERNATING CURRENTINTO 3,524,102 8/1970 Michalski et al ....3l5/240 X DIRECT CURRENT3,037,147 5/1962 Genuit et al. ....315/240 X 3,538,418 11/1970 Allmgton..321/18 [72] lnventors: Kenzo Tatematsu, Amagasaki; Teruhisfl 3,383,5795/ 1968 Hung ..321/24 X Kaneko, Kadoma, both of Japan [73] Assignee:Matsushita Electric Industrial Co., Ltd., Primary Beha Osaka, JapanAttorney-Stevens, Davis, Miller 8!. Mosher [22] Filed: June 15, 1970[57] ABSTRACT [21] APPl- 46,322 A converter for changing alternatingcurrent into direct current by charging a large-capacity capacitorthrough a full wave [30] Foreign Application priority Dam rectificationcircuit which, together with a transformer, is connected to an AC powersupply. The output voltage of a voltage June 20, 1969 Japan ..44/49863divider circuit consisting f the above memioned capacitor June Japan-W"44/49864 and a parallel-connected resistor is compared with a predeter-Aug. 29, 1969 Japan ..44/69483 mined reference voltage by a comparatorcircuiL Then, if the former is lower than the latter, a gate circuit istriggered to put [52] US. Cl ..321/14, 315/240, 321/ 1 8, a semoscmatingcircuit into a State where it is ready for oscil SI I t g 3 lation, andthe oscillating operation of the above-mentioned d 383 self-oscillatingcircuit is triggered through the gate circuit con- 1 0 nected to anintermediate point of a closed loop made up of a rectification circuitfor rectifying an AC source voltage and of 56] Reierences Cited a partof the aforesa d voltage dlvrder circu t, the output of theself-oscillating circuit providing a trigger signal for a thyristorUNITED STATES PATENTS means connected to the aforementioned transformer.

3,375,403 3/1968 Flieder ..323/24 14 Claims, 10 Drawing FiguresRES/$722? L040 /8 W L /4 5b 0 0-! R66 IMR H 7 l3 mar/ 5mm 1 u l I M?RES/570R Ear/FER 6011345470? I56 RJWER TRANS 22%, GATE [MR PHASE 1E7 P-qV50 RES/57D? 7 C PATENTEDFEB29 m2 SHEET 2 BF 7 CONVERTER FOR CHANGINGALTERNATING CURRENT INTO DIRECT CURRENT The present invention relates toa converter for changing alternating current into direct current, ormore particularly to a converter in which a DC output voltage isimpressed on a large-capacity capacitor to temporarily store energywhich is repeatedly supplied to a load at frequent intervals during ashort period of time.

As an AC to DC converter of this sort, a copying machine employing axenon lamp light source is known. Although it can produce a largeluminous flux, a xenon lamp draws considerable electric current and itis more advantageous to employ such a lamp through which a large currentflows during a very short period of time.

It is also known that, in those machines and apparatus in which alarge-capacity capacitor is impressed with a DC output, a large inrushcurrent flows from the power source to the machines and apparatus duringthe initial stage of charging the capacitor. This surge current has avery great impact on the components of the machines and apparatus,resulting in their damages or, in worst cases, destruction. As a meansfor controlling the surge current, a phase control with a thyristor iscommonly used.

An object of the present invention is to provide an AC to DC converterto which is commercially applied a novel controlling means forcontrolling the firing angle of a thyristor in accordance with thecharging state of a large-capacity capacitor to effectively prevent thesurge current from flowing into the capacitor.

Another object of the present invention is to provide an AC to DCconverter capable of manually regulating the time to end the charging ofa large-capacity capacitor.

A further object of the present invention is to provide an AC to DCconverter which, in the event of malfunction of its means for setting acapacitor-charging voltage, prevents the charging to an excessivevoltage and secures operator safety without affecting other components.

A still further object of the present invention is to furnish an AC toDC converter which is capable of stopping the capacitor chargingoperation when a thyristor for smoothly starting the machine breaks downand which is thus capable of preventing an extension of the trouble.

A still further object of the present invention is to provide an AC toDC converter suitable for a device to turn on a discharge lamp which hasa high starting voltage and requires a large current for actuation.

The above and other objects, features and advantages will be madeapparent by the detailed description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a drawing showing a construction of the present invention, asbased on its principle;

FIG. 2 illustrates an embodiment of the present invention;

FIG. 3 is a diagram showing the operation of the embodiment shown inFIG. 2;

FIG. 4 shows another modification of the invention; and

FIGS. 5a to 5f indicate modifications of the essential parts of theabove.

A detailed explanation of the present invention, starting with theprinciple of its construction, is given below.

FIG. 1 shows a construction based on the principle of this invention,wherein a main circuit consists of a main power source transformer l, athyristor 2 and a choke coil 3 connected in series with the primary coilof the transformer, a full wave rectifier 4 made up of diodes connectedwith the secondary coil of the transformer, a large-capacity capacitor 5impressed with the output of the full wave rectifier and a load 6supplied with the energy stored in the capacitor 5.

The phase control circuit of the thyristor 2 is constructed in such away that a self-oscillator circuit 10 producing a trigger signal for thethyristor is controlled by a gate circuit 9 which in turn is controlledby three voltages, i.e., the voltage of a voltage divider circuitconsisting of resistors 7a, 7b and 7c"which divide the charging voltageof the capacitor 5, the voltage obtained by shifting the phase of thesource voltage by a certain angle and the output voltage of a comparatorcircuit 8 which compares the charging voltage of the capacitor 5 or itsequivalent with a reference voltage.

The voltage whose phase is shifted by a certain angle with respect tothe phase of the source voltage can be obtained by adding a phaseshifting circuit 12 to the secondary side of a power transformer 11 forthe control circuit. A rectification circuit 13 is also added to thesecondary side of the aforementioned power transformer 11 to supply thepower required for the control circuit.

On the other hand, the comparator circuit 8 is connected with anovercharge-protecting circuit 14 which is energized when a voltageappearing at the sliding terminal of the resistor 7b as a result ofdividing the charging voltage of the capacitor 5 exceeds a predeterminedvalue, thereby stopping the charging action. The capacitor 5 is thuscontrolled so that its rated breakdown voltage is not exceeded.Incidentally, 15a, 15b and show impedance elements.

The operation of the aforesaid components is explained below. The powertransformers l and 11 are connected across the power source 18 through aswitch 16 and a fuse 17. Immediately after the switch 16 is closed, thecapacitor 5 has not been charged and no voltage appears across theresistors 7a, 7b and 70. When the input voltage of the comparatorcircuit is lower than a reference voltage, the gate circuit 9 makes theself-oscillating circuit 10 ready to oscillate.

The output voltage of the phase-shifting circuit 12, on the other hand,is supplied to the gate circuit 9 to trigger the selfoscillating circuit10 which is ready to oscillate. The time to start oscillation can becontrolled and consequently the conduction can angle of the thyristor 2determined by constructing the gate circuit 9 is such a manner that theself-oscillating circuit 10 is triggered at a certain level of thephase-shifted voltage.

When conduction through the thyristor 2 begins at a small conductionangle, an exciting current flows into the main power transformer I,inducing in its secondary coil a boosted AC voltage, which after beingrectified by the rectification circuit 4 is impressed on the capacitor 5to charge it. When the capacitor 5 starts to be charged, the comparatorcircuit 8 and the gate circuit 9 are impressed with a voltage or controlsignal through the resistors 7a, 7b and 7c. Across the resistor 70 comestoexist a DC voltage, which changes the level of the output supplied bythe phase shifter circuit 12 to the gate circuit 9, thereby advancingthe firing phase of the thyristor 2 and enlarging its conduction angle.As a result, the electric current flowing into the main powertransformer l is increased, expediting the charging of the capacitor 5.

When the voltage which is obtained by dividing the charging voltage ofthe capacitor 5 and which is impressed on the comparator circuit 8reaches the level of its reference voltage, the comparator circuit 8reverses its output condition,'actuating the gate circuit 9 to stoposcillation of the self-oscillating circuit 10.

By the way, it is for the following reason that the phase shiftercircuit 12 is added. With the increase in energy supplied to a load, thecapacity of the main power transformer l is enlarged, resulting in anincreased exciting current. Therefore, an inrush exciting currentseveral to 10 or more times as large as the current under normalconditions flows through the primary coil of the main power transformer1, depending on the conduction phase of the thyristor 2. Also, thecharging current of the large-capacity capacitor 5 combines with theaforesaid exciting current to cause a very large surge current.

In a system for controlling the load by using the thyristor 2 at theprimary side of the main power transformer l, as the load frequencybecomes higher, a large exciting current flows more frequently. Sincethis surge exciting current is delayed by about 1r/2 with respect to thevoltage phase, it is necessary to select the triggering phase of thethyristor 2 at zero or a very ,small value of the current phase. Thephase shifter circuit 12 is for determining the triggering phase of thethyristor 2.

lt is undesirable from the viewpoint of both performance and economy notto control the initial triggering phase of the thyristor 2. In the firstplace, it is required that the excess current capacity of the thyristor2 be made large so that it can sufficiently withstand the surge of theexciting current. This calls for an electric current capacity largerthan needed for each element and contributes to a higher cost of theapparatus. Secondly, the flow of a large exciting current temporarilyputs the devices connected to the AC power source 18 in a state of lowimpedance or short circuit, which has a bad effect on not only the powersupply, but also other devices connected to it. Thirdly, the protectivemeans such as a thermal nofuse breaker and a fuse which are insertedbetween the AC power source and other devices are liable to be energizedeven by an instantaneous excess current. For this reason, it oftenhappens that the devices are inconveniently cutoff from the AC powersource by the protective means. Fourthly, not only the thyristor 2 butalso other components must be improved in their characteristics towithstand the surge current, resulting in a still higher cost.

In view of the above, the initial triggering phase of the thyristor 2should be controlled.

The gate circuit 9 is provided for the following reason: As describedabove, the inrush magnetizing current into the main power transformer 1lags behind the phase of the AC source voltage by about 11/2. Therefore,by regulating the phase shifting circuit 12 in such a way that theelectric current flows through the thyristor 2 in the above-mentionedphase condition, the exciting current of the power transformer 1 and thecharging current to the capacitor can be controlled. When the conductionangle of the thyristor 2 is maintained at a fixed level, however, theratio of the firing period to one cycle of the output of the AC powersource 18 is small, holding the charging current of the capacitor 5 at asmall value. Because of the small value of the charging current, ittakes a long time until the charging voltage of the capacitor 5 reachesthe predetermined voltage value. This reduces the operating efficiencyof the apparatus. There is also a way of providing a phase control atthe first half cycle, supplying electricity without any subsequent phasecontrol. Although this method is advantageous in that the charging timeof the capacitor can be shortened, there is still room for improvementsto be made, because only a small amount of charge can be stored at thefirst half cycle and a large current flows at the next half cycle.

The problems mentioned above are soluble by increasing the firing angleof the thyristor 2 at each half cycle. For this purpose, the level ofthe output voltage of the phase shifting circuit 12 must be changed withthe increase in the charging voltage of the capacitor 5, and also ameans must be provided so as to control the oscillation-starting phaseof the self-oscillating circuit 10 in accordance with a certainrelationship between the above two voltages. The gate circuit) isrequired to simultaneously control the self-oscillating circuit 10 bymeans of a plurality of control voltages mentioned above.

The charging voltage of the capacitor 5 is determined by the positionofthe sliding terminal of the resistor 7b. Therefore, the resistor 7b isused with relative frequency and its structural reliability is lowerthan that of other component parts. Because this resistor 7b is a factordetermining the charging voltage of the capacitor 5, some other means inplace of it must be secured in case of a failure. Here in thisinvention, it is so constructed that the voltage across the terminals ofthe resistor 7c is led to the overcharging protecting circuit 14 by theimpedance element 15b to cut off the apparatus from the AC power source18 when the charging voltage of the capacitor 5 reaches an allowablelimit.

FIG. 2 is a diagram showing an electric circuit of a copying machineembodying the present invention. Closing the switch SW, causes theelectric current to flow through the power transformer T, for acontrolling circuit to which DC power is supplied through therectification circuit D and the capacitor C The phase of the secondaryvoltage of the transformer T is advanced by approximately 1r/2 withrespect to the voltage of the power source S by the capacitor C It isthen rectified by the full wave rectifying circuit D,, to be convertedinto a signal for controlling the thyristor TH. In addition, thecapacitor C,, a component used in generating an exciting voltage for thelamps La, and La, is charged through the diode D,,.

At this juncture, a switch SW closed to the side of the terminal acauses the base and emitter of the transistor 0-,, to beshort-circuited, thus keeping it in a cutoff state. At the same time,the transistor Q, is maintained in a cutoff state, the base bias voltagebeing supplied to the transistor 0,, to permit the current to pass it.Since the capacitor C is short-circuited by the transistor Q theself-oscillating circuit consisting of the capacitor C resistor R, andthe unijunction transistor 0,, is not actuated. As a result, no gatesignal is impressed on the bidirectional triode thyristor Tl-l throughthe pulse transformer T and the capacitor C, is not charged.

If the switch SW is closed to the side of the terminal b, the transistor0;, is made ready to operate. The capacitor C, is not chargedimmediately after the closing of the switch SW and therefore the voltageobtained from the resistors R,, VR,, VR VR, and VR, and the diode D,equals zero. That is to say, the sliding terminal of the resistor VR,,the contact between the resistors VR, and VR, and the sliding terminalof the resistor VR, are maintained at ground potential. Because thesliding terminal of the resistor VR, and the node of the resistors VR,and VR, are maintained at ground potential, the bases of the transistorsQ and Q, are kept at ground potential through the diodes D and D and theresistor R respectively. Thus, the transistors Q and Q, maintain acutoff state. The transistor Q which is kept at a cutoff state by thetransistor 0,, causes the current to flow through the transistor Qputting the transistor 0, into a state ready for conduction.

The base potential of the transistor Q, varys with the output of thephase shifting circuit consisting of the capacitor C and the full waverectifying circuit D,,.

Referring to FIG. 3 where the waveforms of the voltage and current atthe various parts are shown, a full wave rectifying voltage E, which isadvanced in phase by about 11/2 ahead of the voltage E, of the AC powersupply S appears at the ungrounded terminal of the full wave rectifyingcircuit D,,. As the terminal on the positive side of the full waverectifying circuit D,, is grounded, the voltage E, becomes negative asshown in the drawing.

Watching the half cycle of the AC voltage E, immediately after closingthe switch SW to the side of the terminal b, the voltage E, stands atzero in the neighborhood of the phase rr/2 of the AC voltage. In acircuit including the resistors R and R and the variable resistor VR,which are connected in series with the full wave rectifying circuit D,,,a voltage E or a division of the voltage E,, appears at the node of theresistors R and R The voltage E is also elevated to zero in theneighborhood of the phase 11/2 of the source voltage E Consequently, thecathode potential of the diode 5 is highest at the aforesaid phase, andthe base potential of the transistor 0., is also increased.

As described above, since the current flows through the transistor 0,,by changing the position of the switch SW the current also passesthrough the transistor 0, while the cathode potential of the diode D isabout zero. Due to the conduction of the transistor 0., as mentionedabove, the transistor O is reversed to remove the short circuit acrosscapacitor C As a result, a charging current flows into the capacitor Cthrough the resistor R When the voltage across the terminals of thecapacitor C reaches the firing voltage for the unijunction transistor0,, conduction begins through the transistor Q permitting a pulse-shapedcurrent to flow in the primary coil of the pulse transformer T The pulsevoltage E is induced in the secondary coil of the pulse transformer Tand this pulse voltage is impressed on the gate electrode of thebidirectional triode thyristor TH as a trigger signal, thereby enablingthe current to flow through the thyristor TH.

rent I flows through the main power transformer T, to start the chargingof the capacitor C,. The voltage E, across its terminals comes up alittle as shown in the drawing.

On the other hand, the oscillating circuit consisting of the unijunctiontransistor 0,, the resistor R and the capacitor C oscillates at afrequency determined by the time constant C R,

and the firing voltage of the unijunction transistor 0,. The voltage Eis reduced in accordance with the output voltage E, of the full waverectifying circuit D,,, lowering the cathode potential of the diode DTherefore, the bias voltage between the base and the emitter of thetransistor Q, is also reduced, cutting off the transistor. The reversingof the transistor 0., causes the conduction of the transistorterminating the oscillation of the aforesaid oscillating circuit.Conduction through the thyristor TH continues until the current Ibecomes zero even if the voltage E ceases to exist. When the current Ibecomes zero, the thyristor TH is restored to a cutoff state.

When the capacitor C, is charged, the voltage corresponding to thecharging voltage appears at the sliding terminals of the resistors VR,and VR,. The voltage generated at the sliding terminal of the resistorVR, is impressed on the transistor Q, through the resistor R and, in thetransistor 0,, is compared with the breakdown voltage of the zener diodeZD. The same can be said of the transistor 0-,.

Also, the potential of the sliding terminal of the resistor VR, iselevated a little higher than the ground potential, increasing theconduction angle of the thyristor TH at the next half cycle of the ACsource voltage E,.

In .other words, while the phase of voltage E, is between 1r and Zr, thevoltage IE at the node of the resistors R and R is made higher in theneighborhood of the phase 3rr/2 than at the phase 1r/2. Hence the firingof the transistor 0., is advanced and consequently, the time to startthe oscillation of the oscillating circuit consisting of the unijunctiontransistor Q61 the resistor R, and the capacitor C is advanced. Thisincreases the conduction angle of the thyristor TH, lengthening theperiod during which the charging current for the capacitor C, flows.

Each time the capacitor C, is charged, the conduction angle of thethyristor is enlarged and the current I is increased, while being socontrolled that it does not become excessively large, thereby increasingthe charging voltage E, of the capacitor C According as the chargingvoltage E, increases, the voltage at the sliding terminal of theresistor VR, is heightened. If this voltage goes higher than the sum ofthe breakdown voltage of the zener diode ZD, the voltage drop at thebase-emitter junction of the transistor 0, and the voltage drop in thediode D conduction through the transistor Q, begins. Then, conductiontakes place in the transistor 0,, the transistors Q and Q, are cutoff,the current starts to flow through the transistor 0,, and the capacitorC,, is short-circuited, terminating the oscillation. The above gatingoperation of the transistor Q, is secured by the existence of the diodeD Changing the position of the switch SW from the terminal a to theterminal b on completion of charging the capacitor C,, the dischargingof the capacitor C causes the gate current to flow into the siliconcontrolled rectifier SCR,, starting the flow of electric current throughit. Then, the capacitor C,, which hasbeen charged through the diode D isdischarged through the primary coil of the pulse transformer T inducinga pulse voltage in its secondary coil.

The voltage induced in the pulse transformer T, is impressed on thetrigger coil of the lamps La, and La,, causing discharge thereof. Assoon as a faint discharge occurs across the trigger coil and theelectrode of the lamps La, and L11 a main discharge starts, causing thecapacitor C, to be discharged through the lamps La, and La and the chokecoil CH The lamps La, and La, then radiate, consuming the energy storedin the capacitor C Due to the discharge of the capacitor C,, the voltageE becomes approximately zero, repeating the above-mentioned chargingoperation.

At the time when the charging voltage E, of the capacitor C, reaches apredetermined value and conduction through the transistor Q, starts, thetransistor 0, which has an emitter circuit common to the transistor 0,is in a cutoff state. This is because of the function of the diode Dbetween the sliding terminal of the resistor VR, and the base of thetransistor 0-,, and the firing voltage of the transistor 0 issubstantially elevated by the amount of voltage drop in the diode D Thefollowing is an explanation of the above apparatus in the case of afailure of the above means. The resistor VR, is provided for the purposeof determining the charging voltage E of the capacitor C,, and is usedon relatively many occasions. Therefore, contacts troubles are liable todevelop between its sliding terminal and the resistance material,frequently resulting in the loss of the resistor function.

When the resistor VR, fails, the comparison operation by the transistorQ, stops and the charging of the capacitor C, continues with the resultthat the charging voltage is apt to exceed an allowable limit thereof.Under this condition, the voltage at the node of the resistors VR, andVR, is impressed upon the transistor 0 through the diode D Conduction ofthe transistor 0, causes another conduction of the transistor 0,,thereby triggering the silicon controlled rectifier SCR,. When currentstarts to flow through the silicon controlled rectifier SCR, the relayR, is excited to open the switch SW, between the power transformer T,and the power source S, separating the apparatus from the power sourceS. At the same time, the collector current of the transistor Q, passesfrom the diode D, through the resistor R,,, triggering the siliconcontrolled rectifier SCR,. Thus, the capacitor C, is discharged throughthe lamps La, and L11 Even if the resistance material of the resistorVR, is worn out or disconnected due to the movement of the slidingterminal, entirely the same protective function as mentioned above issecured by the variable resistor VR, connected in parallel with theresistor VR,. Needless to say, an ordinary fixed resistor instead of thevariable resistor VR, may be used.

Moreover, contacts faults or other breakdowns occurring in the resistorVR, causes the electric current from the full wave rectifying circuit D,to flow through the ground, diodes D and D and the resistor R in thatorder. The transistor Q, is reversely biased by the voltage across theterminals of the diode D and cutoff, making the current flow through thetransistor Q When the phase of the source voltage E, equals 1r/2 or11-12 times an odd number, the output voltage of the full waverectifying circuit D goes to zero, which cuts off the diode D andreverses the transistor 0 As a result, in the neighborhood of the abovephase, the oscillating circuit consisting of the unijunction transistor0 the resistor R, and the capacitor C is energized, delivering a gatesignal to the thyristor TI-[ to start its conduction. Since thissequence is repeated in every half cycle of the source voltage E,, noexcess current flows into the apparatus from the AC power source S eventhough the charging time is extended.

Thus, the above means prevents the surge of exciting current which isotherwise observed immediately after the power supply is switched on.This enables the large-capacity capacitor C, to be charged in a shortperiod of time. The charging voltage E, and the charging time for thecapacitor C, can be regulated by the resistors VR, and VR, respectively.In addition, since capacitor C, is not charged to an voltage, greatersafety is assured.

FIG. 4 is a drawing which illustrates a similar circuit to that shown inFIG. 2, and in which the control of the firing angle of the thyristor THis revised. The following is an explanation of the revision.

The full wave rectifying circuit D is connected at the secondary side ofthe transformer T to charge the capacitor C,,, through the resistorR,,,, and the variable resistor VR,,,,. Until the capacitor C, reaches apredetermined value of voltage, the transistor Q,,,,corresponding to thetransistor O in FIG. 2is in a cutoff state. Also, no voltage appears atthe sliding terminal of the resistor VR,,, -corresponding to theresistor VR, in FIG. 2and the transistor Q|02 is cutoff. Therefore, asmoothing circuit consisting of the resistor R and the capacitor C iselectrically isolated from the output terminals of the full waverectifying circuit D and a full wave rectified voltage is directlyimpressed on the capacitor rot- At this time, if the breakover voltageof the unidirectional diode thyristor D inserted between the capacitor Cand the primary coil of the pulse transformer T is set at a valueequivalent to or a little lower than the crest value of the full waverectification voltage, the capacitor C is discharged at the phase ofabout 1r/2 later than the AC voltage through the aforesaidunidirectional diode thyristor D and the primary coil of the pulsetransformer T This discharge induces a pulse voltage in the pulsetransformer T triggering the thyristor TH.

The discharging of the capacitor C, remarkably reduces the voltagebetween its terminals and restores the unidirectional diode thyristor Dto a cutoff state. This oscillation frequency being determined by thecapacitor C the pulse transformer T and the resistors R and VR thecharging and discharging are repeated.

At the initial stage of switching on the power supply, the transistor Qis in a cutoff state and no smoothing operation is performed by thecapacitor C and the resistor R Therefore, the voltage impressed on thecapacitor C is in the form of a pulsating current and is reduced lowerthan the breakover voltage of the unidirectional diode thyristor D in ashort time, thereby stopping the oscillating action. On the other hand,the thyristor TH continues conduction until the current fed by the ACpower source S comes to zero, but due to the smallness of its conductionangle, an excessive surge current can be prevented.

After the capacitor C is charged because of the conduction of thethyristor TH, a voltage presents itself at the sliding terminal of theresistor VR This voltage improves the conductive state of the transistorO with a result of electrically inserting the capacitor C and theresistor R between the output terminals of the full wave rectifyingcircuit D Hence, the smoothness of the rectification voltage isimproved, the oscillation-starting phase is advanced, and the conductionangle of the thyristor TH is increased.

In other words, early in the stage of charging the capacitor C,, thetransistor is cut off or in a nearly cutoff state and the unidirectionaldiode thyristor D is impressed with a voltage which increases with thefull wave rectification voltage. The unidirectional diode thyristor D isthus put into a conductive state in the neighborhood of a crest value ofthe impressed voltage. Due to this conduction, the voltage across theterminals of the capacitor C drops and conduction again takes place nearthe crest value of the next half cycle. However, the charging of thecapacitor C, improves the conduction of the transistor Q electricallyinserting the capacitor C, and the resistor R Hence the DC portion ofthe voltage impressed on the capacitor C is increased, which advancesthe firing phase of the unidirectional diode thyristor D increasing theconduction angle of the thyristor TH.

When the charging voltage of the capacitor C, reaches a prescribedvalue, conduction of the transistor Qrm begins and the capacitor C isshort-circuited. When the charging of the capacitor C is stopped, novoltage is impressed on the unidirectional diode thyristor D, withoutgenerating any signal for triggering the thyristor TH. Therefore, thecharging operation of the apparatus stops.

FIG. 5 shows examples of a partial modification, in which portionscorresponding to those of the apparatus shown in FIG. 2 have the samedesignation. First of all, FIG. 5a shows a modification of the thyristor2 for controlling the surge current, using the silicon controlledrectifiers SCR and SCR connected in parallel in opposite directions. Asa load for the unijunction transistor Q,;, the pulse transformer T, withthree coils is used, one of the coils being connected to the unijunctiontransistor 0 and the other two to a point between the gate and thecathode of the silicon controlled rectifiers SCR and SCR through theresistors R and R respectively.

The silicon controlled rectifiers SCR and SCR are made conductive when aforward direction is given at every half cycle.

FIG. 5!: illustrates an instance where the two-terminal bidirectionaldiode thyristor TH is provided at the primary side of the transformerT,. Taking advantage of the breakover of the unidirectional diodethyristor D in place of the unijunction transistor Q a gate signal isimpressed across the terminals of the above-mentioned thyristor TH fromthe pulse transformer T through the DC blocking capacitor C Theoperation of this device, as in the case of the device shown in FIG. 5a,is equivalent to that of the device shown in FIGS. 2 and 4.

FIG. 5c shows a modification of the input section of the gate circuit orthe comparator circuit 8 and the preceding stages for detecting thecharge voltage of the capacitor 5. To begin with, the charging voltageof the capacitor C, is divided by the resistors R and R and then thevoltage division is impressed on the transistor Q in a emitter-followercircuit. The

emitter circuit of the transistor Qm Consists of the resistors VR,, VRand VR;, and the diode D,. Voltage impression is made from the slidingterminal of the resistor VR, through the resistor R on the base of thetransistor O which has a zener diode ZD as an emitter load. Thistransistor Q together with the transistor Q makes up a Schmitt circuit,controlling the conduction of the transistor Q and subsequent elementsconnected from the output of the circuit.

The employment of the emitter-follower circuit makes it possible toprevent an erroneous actuation due to noises, even if the design of theapparatus requires the resistor VR. to be connected with a lead wirefrom a control circuit which is located remote from the resistor.

It goes without saying that the Schmitt circuit consisting of thetransistors Q40: and 040:; may be replaced by a construction employingthe transistor 0, as shown in FIG. 2.

FIG. 5d shows the case in which the reverse blocking triode thyristor THis used instead of the silicon controlled rectifier SCR in FIG. 2,without the transistor 0,, and an accompanying circuit.

FIG. 52 illustrates a modification of a combination of the gate circuit9 and the self-oscillating circuit 10, wherein, instead of thetransistor 0;, in parallel with the capacitor C the transistor 0connected in series with the resistor R is controlled by the collectoroutput of the transistor Q The operation in this case is equivalent tothat in FIG. 2.

FIG. 5f is an instance in which an astable multivibrator is used for theself-oscillating circuit H0. The stable multivibrator consists of thetransistors Q and O and its power supply is controlled by the transistorQ which in turn is driven by the transistor Q The output of the astablemultivibrator energizes the switching transistor O and a gate signal isdelivered to the thyristor TH through the pulse transformer T What weclaim is:

1. A converter for changing alternating current into direct currentcomprising, a transformer for changing an AC source voltage, a reactorand a thyristor means attached to said transformer, a first rectifiercircuit for rectifying the output of said transformer, a capacitor whichis charged by the output of said rectifier circuit and which produces aDC voltage, a voltage divider circuit which produces a voltagecorresponding to the charging voltage of said capacitor, a comparatorcircuit for comparing the output voltage of said voltage divider circuitwith a predetermined reference voltage, a gate circuit which isenergized in accordance with the output of said comparator circuit as afirst controlling signal, an oscillator circuit whose oscillating actionis controlled by said gate circuit and which supplies a trigger signalto said thyristor and a second rectifier circuit which rectifies the ACsource voltage to obtain a second controlling signal which the gatecircuit combines with a third signal to change the oscillation startingphase of said oscillator circuit, said third controlling signal beingobtained from said voltage divider circuit and corresponding to thecharging voltage of said capacitor, said gate circuit being opened bythe first controlling signal when the charging voltage of said capacitoris lower than a predetermined reference voltage of said comparatorcircuit, said second controlling signal being led to said oscillatorcircuit in combination signal with the third controlling signal which issubject to change with the increase in the charging voltage of saidcapacitor, thereby advancing the oscillation starting phase of saidoscillator circuit with respect to the AC source voltage to increase theconduction angle of said thyristor means.

2. A converter for changing alternating current into direct currentaccording to claim 1, wherein a phase shifter circuit is provided byadding a capacitor to said second rectifier circuit and the phase of thesecond controlling signal is shifted about 7r/2 from the phase of the ACsource voltage.

3. A converter for changing alternating current into direct currentaccording to claim 2, in which is employed said voltage dividing meanshaving as a voltage divider circuit resistor in parallel with thecapacitor charged by said first rectifier circuit, said phase shiftercircuit and a part of said voltage dividing means forming a closed loopfrom an intermediate point of which a combined signal of said second andthird controlling signals is impressed on said gate circuit.

4. A converter for changing alternating current into direct currentaccording to claim 2, wherein said oscillator circuit has a unijunctiontransistor, a resistor inserted between the emitter and one of the basesof said transistor, a capacitor inserted between the emitter and theother of the bases of said transistor and a pulse transformer which is aload of said transistor; said gate circuit having a base circuitincluding a transistor of said comparator circuit which produces thefirst controlling signal, a closed loop consisting of said phase shiftercircuit and a part of said voltage dividing means, and also a transistorswitching means in parallel with the capacitor of said oscillatorcircuit; and wherein the second and third controlling signals change,with the increase in the DC output voltage, the base bias voltage of thetransistor switching means of said gate circuit impressed with the firstcontrolling signal.

5. A converter for changing alternating current into direct currentaccording to claim 2, wherein said comparator circuit is furtherequipped with an overcharge-protecting circuit for disconnecting the ACpower supply when the output voltage of said voltage divider circuit iscompared with and exceeds the reference voltage by said comparatorcircuit; said voltage divider circuit consisting of a resistor with asliding terminal and a variable resistor connected in parallel; a diodebeing inserted between the sliding terminal of said resistor and theinput terminal of said overcharge-protecting circuit; and another diodebeing inserted between one of the fixed terminals of said resistor andthe input terminal of said overcharge-protecting circuit.

6. A converter for changing alternating current into direct currentaccording to claim 1, wherein said voltage dividing means includesresistors in parallel with said capacitor; and said second rectifiercircuit and a part of said voltage dividing means forming a closed loop,from an intermediate point of which a combination of said second andthird controlling signals is supplied to said gate circuit.

7. A converter for changing alternating current into direct currentaccording to claim 6, wherein said oscillator circuit consists of acapacitor connected to the output terminal of the second rectifiercircuit, a unidirectional diode thyristor connected in series with apulse transformer and a smoothing circuit connected in series with atransistor whose conduction is controlled by the second and thirdcontrolling signals; conduction of said transistor being improved withthe increase in the charging voltage of the capacitor connected with thefirst rectifier circuit; said smoothing circuit making the output ofsaid second rectifier circuit smoother; and thereby controlling theconduction angle of the thyristor connected to said pulse transformer.

8. A converter for changing alternating current into direct currentaccording to claim 1, wherein said comparator circuit is furtherequipped with an overcharge-protecting circuit which cuts off theconnection with the AC power supply when the output voltage of thevoltage divider circuit exceeds the reference voltage of said comparatorcircuit after making a comparison between the two; said voltage dividercircuit consisting of a voltage dividing means including a resistor witha sliding terminal connected in parallel with a variable resistor; adiode being inserted between the sliding terminal of said resistor andthe input terminal of said overchargeprotecting circuit; and anotherdiode being inserted between a fixed terminal of said resistor and theinput terminal of said overcharge-protecting circuit.

9. A converter for changing alternating current into direct currentaccording to claim 1, wherein a controlled rectifying means comprisingtwo silicon controlled rectifiers connected in parallel in oppositedirections is used as a thyristor means; and the output of saidoscillator circuit being supplied to said silicon controlled rectifiersas a gate signal.

10. A converter for changing alternating current into direct currentaccording to claim 1, wherein said thyristor means is a bidirectionaldiode thyristor; the output of said oscillator circuit being supplied toa point between the terminals of said bidirectional diode thyristorthrough a DC blocking condenser.

11. A converter for changing alternating current into direct currentaccording to claim I, wherein said thyristor means is a bidirectionaltriode thyristor; the output of said oscillator circuit being suppliedto the gate electrode of said bidirectional triode thyristor.

12. A converter for changing alternating current into direct currentaccording to claim 1, wherein said comparator circuit is a Schmittcircuit; and a constant-voltage conducting diode being connected toemitter circuits of two transistors making up said Schmitt circuit.

13. A converter for changing alternating current into direct currentaccording to claim 1, wherein said oscillator circuit is an astablemultivibrator; a current-controlling means being inserted between saidastable multivibrator and the DC power supply; and conduction of saidcurrent-controlling means being controlled by the gate circuit tothereby control the start and stop of the oscillating action of saidastable multivibrator.

14. A converter for changing alternating current into direct currentcomprising; a transformer for changing an AC source voltage; a thyristorhaving a gate electrode and connected in series with a reactor to theprimary side of said transformer; a first full wave rectifier circuitconnected to the secondary side of said transformer; a first capacitorwhich is charged by said ,rectifier circuit and which produces a DCvoltage; a voltage divider circuit consisting of resistors which dividethe charging voltage of said capacitor, a comparator circuit which has azener diode as it reference voltage source and which compares saidreference voltage with the voltage obtained from said voltage dividercircuit and corresponding to the charging voltage of said capacitor,discharge trigger circuit which jointly uses said reference voltagesource and which releases to a load the charges stored in said capacitorwhen the voltage obtained from said voltage divider circuit reaches thereference voltage; a phase shifter circuit which produces a pulsatingcurrent having a phase about 1r/ 2 shifted from that of the AC sourcevoltage through the second capacitor and the second full wave rectifiercircuit; a gate circuit energized by the output of said comparatorcircuit and the output obtained by combining the output voltage of saidphase shifter circuit with the output voltage of said voltage dividercircuit which increases according as the charging voltage of said firstcapacitor increases; a self-oscillator circuit whose oscillating actionis controlled by said gate circuit and which sends its output to thegate electrode of said thyristor to trigger the same; the output of saidphase shifter circuit impressed on said gate circuit being increased bythe output voltage of said voltage divider circuit as said capacitor ischarged; the phase, in which oscillation of said self-oscillator circuitis started by said gate circuit, thereby being gradually advanced toincrease the conduction angle of said thyristor.

1. A converter for changing alternating current into direct currentcomprising, a transformer for changing an AC source voltage, a reactorand a thyristor means attached to said transformer, a first rectifiercircuit for rectifying the output of said transformer, a capacitor whichis charged by the output of said rectifier circuit and which produces aDC voltage, a voltage divider circuit which produces a voltagecorresponding to the charging voltage of said capacitor, a comparatorcircuit for comparing the output voltage of said voltage divider circuitwith a predetermined reference voltage, a gate circuit which isenergized in accordance with the output of said comparator circuit as afirst controlling signal, an oscillator circuit whose oscillating actionis controlled by said gate circuit and which supplies a trigger signalto said thyristor and a second rectifier circuit which rectifies the ACsource voltage to obtain a second controlling signal which the gatecircuit combines with a third signal to change the oscillation startingphase of said oscillator circuit, said third controlling signal beingobtained from said voltage divider circuit and corresponding to thecharging voltage of said capacitor, said gate circuit being opened bythe first controlling signal when the charging voltage of said capacitoris lower than a predetermined reference voltage of said comparatorcircuit, said second controlling signal being led to said oscillatorcircuit in combination signal with the third controlling signal which issubject to change with the increase in the charging voltage of saidcapacitor, thereby advancing the oscillation starting phase of saidoscillator circuit with respect to the AC source voltage to increase theconduction angle of said thyristor means.
 2. A converter for changingalternating current into direct current according to claim 1, wherein aphase shifter circuit is provided by adding a capacitor to said secondrectifier circuit and the phase of the second controlling signal isshifted about pi /2 from the phase of the AC source voltage.
 3. Aconverter for changing alternating current into direct current accordingto claim 2, in which is employed said voltage dividing means having as avoltage divider circuit resistor in parallel with the capacitor chargedby said first rectifier circuit, said phase shifter circuit and a partof said voltage dividing means forming a closed loop from anintermediate point of which a combined signal of said second and thirdcontrolling signals is impressed on said gate circuit.
 4. A converterfor changing alternating current into direct current according to claim2, wherein said oscillator circuit has a unijunction transistor, aresistor inserted between the emitter and one of the bases of saidtransistor, a capacitor inserted between the emitter and the other ofthe bases of said transistor and a pulse transformer which is a load ofsaid transistor; said gate circuit having a base circuit including atransistor of said comparator circuit which produces the firstcontrolling signal, a closed loop consisting of said phase shiftercircuit and a part of said voltage dividing means, and also a transistorswitching means in parallel with the capacitor of said oscillatorcircuit; and wherein the second and third controlling signals change,with the increase in the DC output voltage, the base bias voltage of thetransistor switching means of said gate circuit impressed with the firstcontrolling signal.
 5. A converter for changing alternating current intodirect current according to claim 2, wherein said comparator circuit isfurther equipped with an overcharge-protecting circuit for disconnectingthe AC power supply when the output voltage of said voltage dividercircuit is compared with and exceeds the reference voltage by saidcomparator circuit; said voltage divider circuit consisting of aresistor with a sliding terminal and a variable resistor connected inparallel; a diode being inserted between the sliding terminal of saidresistor and the input terminal of said overcharge-protecting circuit;and another diode being inserted between one of the fixed terminals ofsaid resistor and the input terminal of said overcharge-protectingcircuit.
 6. A converter for changing alternating current into directcurrent according to claim 1, wherein said voltage dividing meansincludes resistors in parallel with said capacitor; and said secondrectifier circuit and a part of said voltage dividing means forming aclosed loop, from an intermediate point of which a combination of saidsecond and third controlling signals is supplied to said gate circuit.7. A converter for changing alternating current into direct currentaccording to claim 6, wherein said oscillator circuit consists of acapacitor connected to the output terminal of the second rectifiercircuit, a unidirectional diode thyristor connected in series with apulse transformer and a smoothing circuit connected in series with atransistor whose conduction is controlled by the second and thirdcontrolling signals; conduction of said transistor being improved withthe increase in the charging voltage of the capacitor connected with thefirst rectifier circuit; said smoothing circuit making the output ofsaid second rectifier circuit smoother; and thereby controlling theconduction angle of the thyristor connected to said pulse transformer.8. A converter for changing alternating current into direct currentaccording to claim 1, wherein said comparator circuit is furtherequipped with an overcharge-protecting circuit which cuts off theconnection with the AC power supply when the output voltage of thevoltage divider circuit exceeds the reference voltage of said comparatorcircuit after making a comparison between the two; said voltage dividercircuit consisting of a voltage dividing means including a resistor witha sliding terminal connected in parallel with a variable resistor; adiode being inserted between the sliding terminal of said resistor andthe input terminal of said overcharge-protecting circuit; and anotherdiode being inserted between a fixed terminal of said resistor and theinput terminal of said overcharge-protecting circuit.
 9. A converter forchanging alternating current into direct current according to claim 1,wherein a controlled rectifying means comprising two silicon controlledrectifiers connected in parallel in opposite directions is used as athyristor means; and the output of said oscillator circuit beingsupplied to said silicon controlled rectifiers as a gate signal.
 10. Aconverter for changing alternating current into direct current accordingto claim 1, wherein said thyristor means is a bidirectional diodethyristor; the output of said oscillator circuit being supplied to apoint between the terminals of said bidirectional diode thyristorthrough a DC blocking condenser.
 11. A converter for changingalternating current into direct current according to claim 1, whereinsaid thyristor means is a bidirectional triode thyristor; the output ofsaid oscillator circuit being supplied to the gate electrode of saidbidirectional triode thyristor.
 12. A converter for changing alternatingcurrent into direct current according to claim 1, wherein saidcomparator circuit is a Schmitt circuit; and a constant-voltageconducting diode being connected to emitter circuits of two transistorsmaking up said Schmitt circuit.
 13. A converter for changing alternatingcurrent into direct current according to claim 1, wherein saidoscillator circuit is an astable multivibrator; a current-controllingmeans being inserted between said astable multivibrator and the DC powersupply; and conduction of said current-controlling means beingcontrolled by the gate circuit to thereby control the start and stop ofthe oscillating action of said astable multivibrator.
 14. A converterfor changing alternating current into direct current comprising; atransformer for changing an AC source voltage; a thyristor having a gateelectrode and connected in series with a reactor to the primary side ofsaid transformer; a first full wave rectifier circuit connected to thesecondary side of said transformer; a first capacitor which is chargedby said rectifier circuit and which produces a DC voltage; a voltagedivider circuit consisting of resistors which divide the chargingvoltage of said capacitor, a comparator circuit which has a zener diodeas it reference voltage source and which compares said reference voltagewith the voltage obtained from said voltage divider circuit andcorresponding to the charging voltage of said capacitor, dischargetrigger circuit which jointly uses said reference voltage source andwhich releases to a load the charges stored in said capacitor when thevoltage obtained from said voltage divider circuit reaches the referencevoltage; a phase shifter circuit which produces a pulsating currenthaving a phase about pi /2 shifted from that of the AC source voltagethrough the second capacitor and the second full wave rectifier circuit;a gate circuit energized by the output of said comparator circuit andthe output obtained by combining the output voltage of said phaseshifter circuit with the output voltage of said voltage divider circuitwhich increases according as the charging voltage of said firstcapacitor increases; a self-oscillator circuit whose oscillating actionis controlled by said gate circuit and which sends its output to thegate electrode of said thyristor to trigger the same; the output of saidphase shifter circuit impressed on said gate circuit being increased bythe output voltage of said voltage divider circuit as said capacitor ischarged; the phase, in which oscillation of said self-oscillator circuitis started by said gate circuit, thereby being gradually advanced toincrease the conduction angle of said thyristor.